I am trying to figure the best and most cost effective way to insulate beneath an exposed concrete floor. It has been poured on corrugated metal placed on beams. I am getting a price for closed cell foam being sprayed and also wondered if foam board is an option that might work also? If I go with foam what would be the correct thickness? I'm around 3k elevation with the past month temperatures being below normal, made for a nice thermal slab of coldness. Luckily the building wasn't being used so it didn't matter. It's been below freezing for weeks and just finally warmed up some. I'm in the North Carolina mountains for reference to my zone. I'm not sure what I need to do and would appreciate any advice you can give. Thanks If I can resize the photos I'll post a few

Here are some photos of the floor. Not sure why the photos won't show up, never had problems here before.

So there is a room above the slab? Is that why you want to insulate it?

It could have been insulated before the pour...

No it's actually like a bridge with the underneath exposed and the top being inside. I'm not sure why I can't post photos as it would be easy to understand then. I guess I should have said yes, there is a room above the slab.

https://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=North%20Carolina

You're likely either in Climate Zone 4 or a 5/4 (Marine)

Going to go worst case scenario and assume CZ5.

Code calls for an R30. For foam board, you're looking at about 6" to make that.
Now, if the surface is corrugated, that IS going to leave you with air gaps between the surface and the foam. And hand-piecing that together is a pain in the balls.
Also, you've got thermal bridging through the beams too.

In your situation, honestly, I think spray foam might be a better option. It's conformant, so it sits tight to the structure, and you can also coat the beams as well, eliminating the thermal bridging at the same time.

People will talk your head off about "green-ness". All well and good, but performance matters as too.

The price difference between a custom foamboard setup that eliminates the problems with straightforward overlay and a spray foam setup is likely negligible.

Open cell foam sprayed onto the underside of the steel pans would be cheaper & greener per R than closed cell foam. It's about half the polymer per R, and it uses water instead of HFC245fa as the blowing agent. There are some HFO1234ze blown closed cell foams out there that run about R7/inch, but it's still about 2x the amount of polymer per R as half-pound open cell foam.

In my area half-pound foam runs a bit less than 10 cents per R per square foot, compared to 16-18 cents per R-foot for closed cell foam, which half pound polyurethane only a bit more than half the cost of 2lb polyurethane at any given R-value. YMMV

R30 is code min if the insulation is between wood joists under a standard subfloor. Getting to code on a U-factor basis it needs to be less than U0.033 which is R30.3 "whole assembly" which includes the R-value of the concrete, the finish floor, the top & bottom side.

https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.4

Also, what's the square footage of the underside of that floor?

The square footage is roughly 1600 ' . It is not a residential space, it is a commercial space that was used as a restaurant. I'm not sure how they got by without insulating the space, sastifying code. The first time I saw it I knew something had to be done and not to satisfy code but to satisfy common sense. It would literally be like trying to heat the outdoors in the winter. We have someone wanting to rent the space for a micro brewery and this section will be where all the seating is. Talk about cold,feet in the winter. Can someone help me on photos? I have no idea why they won't post.

Apparently the forums doesn't allow the use of hotlinked images through the formatted interface.

HOWEVER, it gives you access to the HTML code underneath and tossing in image tags works.

Post the photos up somewhere you can share them and get the URL to the images.



Create a new post

Paste in the URLs to your images.

http://cox-supergroups.com/IDFP.jpg

Now click on the HTML tag at the bottom of the editing area.

Search for the URLs you posted and surround them in image tags.


http://cox-supergroups.com/IDFP.jpg

becomes...  (Change { } tags to <>)

{img src="http://cox-supergroups.com/IDFP.jpg"}

Then flip back over to the design tab.



Hit SUBMIT.

For commercial buildings in climate zones 3 & 4 (most of NC, except the NW corner from Yancey through Allegany counties) R10 continuous insulation is all that's required under "mass floors", defined as a slab that's 35lbs per square foot or more, for ~150lbs per cubic foot concrete, 25lbs or more for 120lb concrete (which means a 3" or thicker slab always makes it, but a 2" slab doesn't, and 2.5" depends...) See TABLE C402.1.3:

https://codes.iccsafe.org/public/document/IECC2015/chapter-4-ce-commercial-energy-efficiency

So, 2" of closed cell foam (about $2 per square foot) or 3" of open cell (about a buck a square foot) would get you there.

The slab is 6 inches thick poured on corrugated metal on large metal bridge type beams. I got a price of $2.85 and another for $2.39 per sq ft for closed cell foam. Should I look for cheaper prices? I also wonder if radiant heating could be added at this stage under the slab and metal? Too late? It is alleghany county.

You might be able to put up under-slab hydronic with reflector plates.

Not as efficient as in-floor hydronic.

Then foam over it.

You WILL need to be able to properly insulate the SIDES of the slab too. Otherwise you may as well put a gas fired furnace outdoors and hope to god it can warm up the entire planet...

I believe we will be able to get the sides insulated also. I can't believe the builder/owner didn't install radiant when pouring the slab. He missed a great opportunity. The forced air system will be worked hard to keep up, not so much after we insulate the slab. I may not be able to afford a complete radiant system now but I'd like to install the tubing if it's feasible at this point for later installation.

What if I went with 3 inches of open cell and capped it with an inch of closed cell? More r value, less cost? Not sure if the two are compatible like that?

I've never seen closed cell foam installed over open cell under a floor. In theory it could work, but the surface of open cell foam is a much lumpier, less smooth or even substrate to apply the closed cell foam to. Closed cell foam is pretty smooth by contrast.

Radiant UNDER a slab is a generally a bad idea, difficult to control, and would need even higher R. Roth Panels on top of the slab puts ~R1 -R2 between the tubing and slab, and R3 or more between the finish floor and slab, and is much more responsive to changes in load than embedded PEX in the slab, and WAY more responsive than tubing completely under the slab.

Remember what we said about being an R30 under floor?

4 inches, be it open, or closed-cell, or a combination of the two, will not deliver you R30.

Generally, open-cell spray foam clocks in around R3.5-3.6 (conservatively) per inch.
Closed cell generally clocks in around R5-6 per inch (conservatively).

Also, I take it that this area under the slab is actually exposed to the outside?
If so, open-cell is probably not your best bet.


http://www.greenbuildingadvisor.com...losed-cell

I'm cheap so I'll tell you what I would do. Get some recycled foam board. Mechanically fasten to the pan steel then, overspray with poly to seal all joints and edges. I bought some 3.5" reclaimed iso board for $12 per sheet last year that's $.38 per sq ft.

Just linking the two similar posts by OP together:

Radiant Heat Under Slab

Posted By Dilettante on 04 Feb 2018 04:01 AM
Remember what we said about being an R30 under floor?

4 inches, be it open, or closed-cell, or a combination of the two, will not deliver you R30.

Generally, open-cell spray foam clocks in around R3.5-3.6 (conservatively) per inch.
Closed cell generally clocks in around R5-6 per inch (conservatively).

Also, I take it that this area under the slab is actually exposed to the outside?
If so, open-cell is probably not your best bet.


http://www.greenbuildingadvisor.com...losed-cell

But R30 isn't the requirement- it's a commercial building, and it's a mass-floor, which only needs R10 to meet code in that climate zone.

With a radiant floor a bit more than code-min is rational. At 3" any HFO-blown closed cell foam would be closing in on R20, twice the code prescriptive, and that's what's really called for here not 4", along with Roth Panels on top of the slab for the radiant.

Heatlok HFO High Lift would be R25+ @ 4", R19-R20 at 3", which is fine.

https://www.demilec.com/documents/Tech-Library/Heatlok-HFO-High-Lift/Update/Update-2/Heatlok-HFO-High-Lift-TDS.pdf

LaPolla FOAM-LOK 2000 4G would be R27+ @ 4", R20-ish @ 3"

https://lapolla.com/wp-content/uploads/2017/04/FL-2000-4G-TDS-040517.pdf

Roth panels for the radiant tubing would add R3-R4 above the slab for the majority of the area, and a bit of thermal isolation between the tubing and slab to make it more responsive.

[edited to add]

Even if it were a residential building even when ignoring the mass effect benefits it doesn't need more than R19 of continuous insulation to meet IRC 2015 code minimum for zone 4, or more than R28 for zone 5. On a U-factor basis it only needs to duck under U0.047 (R21.3 "whole assembly") or U0.033 (R30.3 "whole-assembly") The combined interior & exterior air-films add up to more than R1, and the 6" slab is good for at least R0.6, plus whatever finish floor materials are used. A shot 4" of closed cell foam (any vendor) + 1" Roth panel would make it for zone 5 no matter what the finish floor was, and 3" closed cell + 3/4" Roth panel would still make it in zone 4.

And that's even ignoring the fact that it's a mass floor.

https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.4

I'd be curious about the space underneath the bridge. What is it's use, Garage? Parking? Wouldn't any type of foam likely require some sort of fir/ignition barrier?